The subject matter disclosed herein generally relates to a differential signal conditioner and, more particularly, to error compensation in a differential signal conditioner.
As shown in FIG. 1, a general differential input signal conditioner 100 includes input signals 104, compensation signals 106, a signal mux 108 selects from amount the signals 104, 106, a difference amplifier 102, an A/D mux 110, an A/D converter 112, and a digital system/processor 114 that provides selection signals and control signals as shown.
General differential input signal conditioners, such as the differential input signal conditioner 100, amplifies the differential input signal, SIG1, and is generally not able to completely reject a common mode voltage signal, SIG1_CMV. The difference amplifier 103 in the general/legacy differential input signal conditioner 100 diagram shown in FIG. 1 is used to amplify the input signal and attempts to reject its common mode voltage. However, as noted above, a real world difference amplifier is non-ideal and produces an error voltage due to the common mode voltage.
The resulting common mode voltage error term can be described using a common mode rejection ratio (CMRR) that is the ratio of the common-mode gain to differential-mode gain. However, the CMRR of an amplifier varies with input signals, temperature, and from device to device and the input signal DC common mode voltage is typically unknown.
For example, in the case of a thermocouple signal, the CMRR error term could be very significant because it only takes approximately 41 micro-volts of CMRR error to cause a 1 degree Celsius temperature error. The CMRR error term on a strain gauge signal is also significant because strain gauge sensors have a nominal non-zero common mode input voltage. For example, a strain gauge sensor with a 10V excitation typically produces a nominal 5V DC common mode output voltage.
Therefore, the CMRR error term can be a significant error in systems that require high accuracy. However, in most legacy differential input signal conditioners, it is not possible to compensate for the CMRR error term, especially if the common mode input voltage is unknown or varies over a wide range.